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1. Field of the Invention
The present invention relates in general to high-speed electronic transistor devices, and more specifically to InP/InGaAs Heterojunction Bipolar Transistors (HBT).
2. Description of the Related Art
The emitter injection efficiency of a bipolar transistor is limited by the fact that carriers can flow from the base into the emitter region, over the emitter junction barrier, when the junction is under forward bias. Such transistors use a lightly doped material for the base region and a heavily doped material for the emitter. The requirement of a lightly doped material for the base region results in undesirably high base resistances and a thick base region. It is known that for high frequency applications it is desirable to have a thin, heavily doped base and a lightly doped emitter. One solution is the heterojunction bipolar transistor. In these transistors the emitter injection efficiency can be increased without strict requirements on doping. Materials commonly used in heterojunction bipolar transistors include the aluminum galium arsenide/galium arsenide (AlGaAs/GaAs) system because of the wide range of lattice matched compositions. It is also known to use a system where indium galium arsenide phosphide (InGaAsP) is grown on indium phosphide (InP).
Lattice matching is well known in the art and refers to matching of the lattice structure and lattice constant for two materials, for example galium arsenide and aluminum arsenide. Special consideration must be taken when depositing a material that has a lattice constant that is significantly different than the material on which it is being deposited. In the prior art, it is known that a thin layer is in compression or tension along the surface plane as its lattice constant adapts to the seed crystal. When this layer is grown very thick however, the layer eventually cannot maintain the compression or tension strain and it will relieve the strain by relaxing. It relaxes to its natural lattice constant. This is the difference between a relaxed layer and a strained layer. The thickness at which a layer begins to relax is referred to as the critical thickness and it depends on the difference in the lattice parameter of the two materials. For indium galium arsenide on indium phosphide there is only one composition of indium galium arsenide that is exactly lattice matched. Since it is very difficult to get the exact match during crystal growth, it is considered in the prior art that if the perpendicular mismatch is less than 0.2%, then the layers are considered to be lattice matched.
In the prior art galium arsenide grown on aluminum arsenide provided a large change in the band gap between the materials with little change in the lattice constant. Because they have similar lattice constants, they are thus easily grown. The system allows for band gap engineering without a designer being constrained by excessive strain or lattice relaxation since the mismatch was just less than 0.2%.
These materials such as described above allow for band gap engineering, which results in various types of desirable devices. In prior art typical heterojunction bipolar transistors are nominally lattice matched to the substrate lattice constant to avoid defects, stress and relaxation of the base material. These effects are harmful to the performance of heterojunction bipolar transistors and limit band gap engineering. Band gap engineering is used to design devices for different optical effects and electronic effects. The heterojunction bipolar transistor may be formed using MOVCD (Metal Organic Chemical Vapor Deposition), a materials science technology used for growing compound semiconductor-based epitaxial wafers and devices. MOCVD technology is also known as OMVPE (Organo-Metal Vapor Phase Epitaxy) and MOVPE (Metal Organic Vapor Phase Epitaxy). Various epitaxial growth techniques are known in the prior art and include LPE (Liquid Phase Epitaxy) VPE (Vapor Phase Epitaxy) and MBE (Molecular Beam Epitaxy). MOCVD is a dominant growth technique behind the major devices and a popular choice of manufacturers involved in high volume production of epitaxial wafers and devices.
It is a drawback of the prior art that the lattice mismatch is to be kept less than 0.2% and thus there is a need in the prior art for a system for band gap engineering, which provides for devices having greater than 0.2% lattice mismatch.
In general terms the present invention is a heterojunction bipolar transistor (HBT), having a substrate formed of indium phosphide (InP), and having emitter, base and collector layers formed over the substrate such that the base layer is disposed between the emitter and collector layers. In one embodiment, the collector layer is formed from InGaAs and is doped n-type. The emitter layer is formed from InP and is doped n-type. The base layer is formed of indium gallium arsenide (InGaAs) and is doped p-type. The composition of the base layer is graded from InxGa1xe2x88x92xAs to InyGa1xe2x88x92yAs, where x is less than 0.515 and where y is less than 0.53 and less than x.
In the present invention the base layer is only under tensile strain as opposed to similar grading techniques, which use a xe2x80x9cstrain compensatedxe2x80x9d grading technique in which the graded layer is partially tensile strained and partially compressively strained. An X-ray rocking curve of the heterojunction bipolar transistor shows that the base material has a varying lattice constant as indicated by the broad curve on the x-ray scan. More specifically, the heterojunction bipolar transistor has a carbon doped tensile strained graded base layer grown by MOCVD.